An organic electroluminescence (organic EL) device is an organic light-emitting diode (OLED) in which the light emitting layer is a film made from organic compounds, which emits light in response to the electric current. The light emitting layer containing the organic compound is sandwiched between two electrodes. The organic EL device is applied to flat panel displays due to its high illumination, low weight, ultra-thin profile, self-illumination without back light, low power consumption, wide viewing angle, high contrast, simple fabrication methods and rapid response time.
The first diode device was reported by Ching W. Tang and Steven Van Slyke at Eastman Kodak in 1987. The device used a two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer. This resulted in reduction of operating voltage and improvement of the efficiency, thereby leading to the current area of organic EL device research and device production.
Typically, the organic EL device is composed of organic material layers sandwiched between two electrodes. The organic material layers include the hole transporting layer, the light emitting layer, and the electron transporting layer. The basic mechanism of organic EL involves the injection, transport, and recombination of carriers as well as exciton formation for emitting light. When an external voltage is applied across the organic EL device, electrons and holes are injected from the cathode and the anode, respectively. Electrons will be injected from the cathode into a LUMO (lowest unoccupied molecular orbital) and holes will be injected from the anode into a HOMO (highest occupied molecular orbital). Subsequently, the electrons recombine with holes in the light emitting layer to form excitons, which then deactivate to emit light. When luminescent molecules absorb energy to achieve an excited state, the exciton may either be in a singlet state or a triplet state, depending on how the spins of the electrons and holes have been combined. It is well known that the excitons formed under electrical excitation typically include 25% singlet excitons and 75% triplet excitons. In the fluorescence materials, however, the electrically generated energy in the 75% triplet excitons will be dissipated as heat for decay from the triplet state is spin forbidden. Therefore, a fluorescent electroluminescence device has only 25% internal quantum efficiency, which leads to the theoretically highest external quantum efficiency (EQE) of only 5% due to only ˜20% of the light out-coupling efficiency of the device. In contrast to fluorescent electroluminescence devices, phosphorescent organic EL devices make use of spin-orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and the internal quantum efficiency of electroluminescence devices from 25% to 100%.
In 2012, a new type of fluorescent organic EL device was developed by Adachi and coworkers. The new organic EL device incorporated the mechanism of thermally activated delayed fluorescence (TADF), which was a promising way to obtain a high percentage of singlet exciton formation by converting spin-forbidden triplet excitons up to the singlet level through the mechanism of reverse intersystem crossing (RISC).
The phosphorescent organic EL device utilizes both triplet and singlet excitions. Cause of longer lifetime and diffusion length of triplet excitions compared to those of singlet excitions, the phosphorescent organic EL device generally needs an additional hole blocking layer (HBL) between the emitting layer (EML) and the electron transporting layer (ETL) or the electron transporting layer with hole blocking ability (HBETL) instead of the typical ETL. The purpose of the use of HBL or HBETL is to confine the recombination of injected holes and electrons and the relaxation of created excitons within the EML, hence the device's efficiency can be improved. To meet such roles, the hole blocking materials must have HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels suitable to block holes transport from the EML to the ETL and to pass electrons from the ETL to the EML. In addition, good thermal and electrochemical stability of the phosphorescent emitting host material is also needed.
The compounds with a 10H-spiro[acridine-9,9′-fluorene] structure have found their applications in organic EL devices, which can be found in U.S. Pat. No. 8,632,893. In addition, the compounds having a structure of 7′,7′-dimethyl-5′,7′,10,11-tetrahydrospiro[dibenzo[a,d][7]annulene-5,13′-inde no[1,2-b]acridine] were used in OLEDs, which was disclosed in KR Appl. No. 1020100029865. In the present invention, we develop a novel polyhetero-aromatic compound having a core structure of 7′,7′-dimethyl-5′,7′-di-hydrospiro[cyclopenta-[1,2-b:5,4-b′]dithiophene-4,13′-indeno[1,2-b]acridine to improve the performance of the organic EL devices.